Don't show MLC form on thread replies

What does this MR do?

Fixes ~bug where the multiline comment form is shown in thread replies.

Closes: #229596 (closed)

Screenshots

Before

After

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content